Week 1 It’s hard to pinpoint the exact day of conception, so the pregnancy calendar is simply set on the first day of your last menstrual period.
Week 2 Your uterus has just shed its lining and is now building a new one to house the fertilized egg or ovum. One of your ovaries is ripening an egg and preparing to release it.
Week 3 (actual conception) A ripened egg is transported through the fallopian tube, where the sperm and egg meet. Your baby’s sex and genetic traits are determined as soon as he or she is conceived.
Week 4 Once the fertilized egg reaches the uterus, implantation ensues, where the egg embeds itself in the uterus lining. The egg’s cells split into two groups: one to form the placenta, the other to form the embryo.
Week 5 As an embryo, your baby is now about the size of an apple seed. Nerve growth begins with the formation of the neural tube, which will become the nervous system. The heart, the circulatory system, and a simple tube that will develop into the intestines, liver, pancreas, and bladder are also forming.
Week 6 The heart has already begun to beat! The umbilical cord, lungs, and digestive tract start to develop, while the body is forming little buds where the arms and legs will grow from. The eyes, ears, and mouth also start to become defined.
Week 7 Limbs start to take shape, but no fingers and toes yet. The brain, eyes’ lenses, nostrils, intestines, pancreas, and bronchi are starting to grow. The umbilical cord has formed, while the lungs and digestive tract continue to develop.
Week 8 Embryo is now the size of a large grape, weighing less than an ounce. The eyes, ears, and nose are taking a more human shape. Elbows, wrists, fingers, and toes begin to form. Embryonic gonads become either testes or ovaries.
Week 9 Bones and cartilage are now starting to develop. Organs, muscles, and nerves start functioning, causing your little embryo to make tiny, spontaneous movements.
Week 10 The embryo is now a fetus. The amniotic sac is formed to protect and provide the fetus with appropriate warmth. Most of the joints—shoulders, elbows, wrists, fingers, knees, ankles, feet, and toes—have developed. If your baby is male, his scrotum forms at this time.
Week 11 Your baby is ready to sound off a louder heartbeat this time, with the use of an instrument called a Doptone or Doppler stethoscope. Ovaries or testicles are fully formed, though it’s still too soon for a sonogram or ultrasound to tell gender.
Week 12 The baby’s head is half its body length, which is about two to three inches long (the back’s length is measured from crown to rump). Almost all organs are formed, and kidneys are starting to secrete urine into the amniotic fluid. Facial features are properly formed and soft nails are growing on the baby’s fingers and toes.